1. Field of the Invention
The present invention relates to an interstand tension controller for controlling the interstand tension of a workpiece being rolled on a continuous rolling mill having a plurality of rolling stands and provided with a looper between the adjacent rolling stands and, more specifically, to an interstand tension controller suitable for application to a hot finishing mill, and capable of satisfactorily carrying out interstand tension control operation without being disturbed by interaction between the tension of the workpiece and the looping angle, having a simple configuration and capable of being easily adjusted.
2. Description of the Related Art
A hot finishing mill has rolling stands and is provided with a looper disposed between the adjacent rolling stands to stabilize the interstand tension of the workpiece. It is important for carrying out stable rolling operation to stabilize the tension of the workpiece that affects directly the size and the shape of the workpiece by the looper and to suppress the variation of looping angle. Two manipulated variables, i.e., the rotating speed of the rolls of the rolling stand and the looping torque, are controlled to regulate the tension of the workpiece and the looping angle. As shown in FIG. 1, a most common interstand tension controller controls looping angle .theta. by regulating the rotating speed of the rolls of an upper rolling stand i or that of the rolls of a lower rolling stand i+1 and regulates the looping torque according to the variation of the looping angle .theta. to adjust the tension .sigma. to a desired value. The tension control performance of this interstand tension controller, however, is not satisfactory because the tension is controlled in an open-loop control mode. Tension and looping angle interact with each other, namely, the variation of tension entails the variation of looping angle, and vice versa. Being unable to deal with interaction between the tension and the looping angle, the conventional interstand tension controller is unable to stabilize the looping angle.
A controller disclosed in Japanese Patent Laid-open No. 59-110410 measures the tension of the workpiece with a load cell or the like installed in a looper, regulates the rotating speed of the rolls of the rolling stand, i.e., a manipulated variable, to regulate the tension by a feedback loop, and regulates the looping torque or the looping speed, i.e., a manipulated variable, to regulate the looping angle by another feedback loop.
Another controller places a precompensator C, which generally is called a cross controller, before a looper characteristic block G that indicates looper characteristics as shown in FIG. 2 to offset the interaction between the tension and the looping angle by the synergetic effect of the precompensator C and the looper characteristic block G.
Integrating optimum regulators disclosed in Japanese Patent Laid-open Nos. 59-118213 and 59-118214 control the operating speed of a looper driving motor, and use, in combination, a feedback operation for feeding back measurable values, i.e., tension, looping angle and operating speed of the looper driving motor, and a main controller that carries out integration to optimize a P-gain index of performance and an I-gain index of performance in a time domain. To obtain a desired control response by this integrating optimum regulator, an optimum control gain must be determined by setting a weighting matrix for a quadratic evaluation function by a trial-and-error method. A previously proposed H-infinity controller is an improvement of the integrating optimum regulator and specifies closed-loop response in a frequency domain to facilitate the design.
However, since the noninteractive interstand tension controller sets an inverse model of a controlled system in the cross controller, the noninteractive interstand tension controller is unable to deal with variations in the characteristics of the controlled system satisfactorily and is incapable of offsetting the effect of a disturbance, such as the variation of the rolling speed.
The integrating optimum regulator and the H-infinity control are difficult to adjust at the site because the integrating optimum regulator and the H-infinity control need a controller having a complicated configuration, an evaluation function must be determined and the parameters of the controller must be designed so as to optimize the evaluation function.